Voltage regulators are used to provide a stable power supply voltage independent of load impedance, input-voltage variations, temperature, and time, and process. Low-dropout (LDO) regulators are generally distinguished by their ability to maintain regulation with small differences between supply voltage and load voltage. The dropout voltage in an LDO is the difference between the output voltage and the input voltage at which the circuit quits regulation with further reductions in input voltage.
A typical voltage regulator includes a reference voltage, an error amplifier circuitry to compare the reference voltage to an output voltage and a series pass transistor (e.g., bipolar or FET), whose voltage drop is controlled by the amplifier to maintain an output voltage at the required value. A supply voltage is provided to a first terminal of the pass transistor and the load voltage produced by the voltage drop is provided at a second terminal of the pass transistor. If, for example, as a load current decreases, causing the output voltage to rise incrementally, an error voltage will increase, the amplifier output will rise, the voltage across the pass transistor will increase, and the output voltage will return to its original value.
When a supply voltage is initially turned on it may ramp up before an amplifier circuit has time to adjust to the initial ramp up.